This invention pertains to an aqueous inkjet ink, in particular to an aqueous inkjet ink comprising an aqueous vehicle, a pigment colorant and a bleed control agent.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. Inkjet printers are equipped with an ink set which, for full color printing, typically comprises a cyan, magenta and yellow ink (CMY). An ink set also commonly comprises a black ink (CMYK).
Bleed of one color into another is a typical problem in ink jet printing because inks have relatively low viscosity and tend to spread especially for inkjet printers having capability of printing three or four primary colors in a simultaneous (or near simultaneous) fashion. Such bleed of one printing liquid into an adjacent printing liquid results in production of indistinct images with a poor degree of resolution.
Various methods have been proposed to prevent bleed of adjacent printing liquids. One method is to apply the two printing liquids at a distance from one another so that no intermingling or mixing of the printing liquids can occur. However, this method produces images of poor resolution. Another method involves a delay in applying the second printing liquid until the first printing liquid is completely dry. This method is disadvantageous due to its inefficiencies. Yet another approach to control bleed is to increase the rate of penetration of the printing liquid into the substrate, but this causes a reduction of optical density.
U.S. Pat. No. 5,488,402 discloses a method for preventing color bleed between two different color ink compositions wherein the first ink is anionic and comprises a coloring agent which includes one or more carboxyl and/or carboxylate groups, and the second ink includes a precipitating agent which is designed to ionically crosslink with the coloring agent in the first ink to form a solid precipitate in order to prevent bleed between the two ink compositions. Multivalent metal salts are disclosed as being useful as the precipitating agent.
U.S. Pat. No. 5,518,534 discloses an ink set for alleviating bleed in multicolor printed elements by employing a first ink and a second ink, each containing an aqueous carrier medium and a colorant with the colorant in the first ink being a pigment dispersion and the second ink containing a salt of an organic acid or mineral acid to alleviate bleed between the first ink and the second ink.
While the use of inorganic or organic salts in one or more inks of an ink set has provided improved bleed property in inkjet printing, a need still exists for improved inkjet ink formulations that provide good print quality without the drawbacks such as ink instability and corrosion to printhead as a result of having high concentration of salts in inks. The present invention satisfies this need by providing compositions having improved bleed property and good optical density.